This application claims the benefit of Korean Patent Application No. 2002-6752, filed Feb. 6, 2002, in the Korean Industrial Property Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an apparatus for forming a color image, and more particularly, to an apparatus for forming a color image using electrophotograpy, reducing a shock occurring when a developing roller contacts a photosensitive drum, and having a developing unit capable of reducing a pressure difference between the developing roller and the photosensitive drum.
2. Description of the Related Art
In an apparatus for forming a color image using electrophotography, such as a digital copying machine, a color laser printer or the like, a predetermined electrostatic latent image is formed on a photosensitive body. Then, the electrostatic latent image is developed with toner, and the developed image is transferred onto paper via a predetermined transfer medium. Thereafter, the transferred image is heated and pressed, thereby being permanently fused on the paper.
Referring to FIG. 1, the apparatus for forming the color image using electrophotography includes a photosensitive drum 10 as a photosensitive body, a charger 11 charging the photosensitive drum 10, a laser scanning unit (LSU) 12 radiating light onto the charged photosensitive drum 10 and forming a predetermined electrostatic latent image, developing units 13 developing the electrostatic latent image with the toner having four colors, such as yellow (Y), magenta (M), cyan (C), and black (K), a transfer belt 14 on which each developed image having one of the four colors is sequentially overlapped, a first transfer roller 14a transferring the developed image from the photosensitive drum 10 onto the transfer belt 14, a second transfer roller 14b transferring the transfer image having four colors overlapped on the transfer belt 14 onto paper, and a fusing unit 15 heating and pressing the paper and permanently fusing the transferred image on the paper.
Four developing units 13-Y, 13-M, 13-C, and 13-K are elastically biased by corresponding springs (not shown) to be apart from the photosensitive drum 10 and selectively move toward the photosensitive drum 10 according to a rotation of cams 13b. A developing roller 13a is disposed on a front end of the developing units to contact the photosensitive drum 10. The apparatus includes a paper cassette 16, a blade 17 cleaning the photosensitive drum 10, an eraser 18, and a transfer path 19 in which paper is exhausted.
In the apparatus for forming the color image using electrophotography shown in FIG. 1, the color image is formed as follows. First, the photosensitive drum 10 is charged by the charger 11, light is radiated on the photosensitive drum 10 by the LSU 12, and then, an electrostatic latent image of the color image to be developed with a first color is formed in a predetermined region of the photosensitive drum 10. For example, if a yellow image having a yellow color is developed first, as shown in FIG. 2, a developing unit 13-Y having the yellow color accesses the photosensitive drum 10 by the rotation of a corresponding cam 13b-Y, and thus the electrostatic latent image formed on the photosensitive drum 10 is developed with a yellow toner.
FIG. 3 is an enlarged view of the photosensitive drum 10 and a developing roller 13a-Y of the developing unit 13-Y in a development operation. Referring to FIG. 3, when a predetermined force is applied to a leaf spring 13c placed at a rear side of the developing unit 13-Y by a rotation of the cams 13b, the leaf spring 13c is displaced by a predetermined distance ▾X, a force is transferred to the developing unit 13-Y, and thus the developing roller 13a-Y contacts the photosensitive drum 10. Toner T stained on an outer circumference of the developing roller 13a-Y is transferred by this contact to the electrostatic latent image of the photosensitive drum 10 contacting the developing roller 13a-Y In this case, in order to increase an amount of a toner per area stained on the photosensitive drum 10, the developing roller 13a-Y rotates at an angular velocity larger than that of the photosensitive drum 10. The developed yellow image is transferred onto the transfer belt 14 through a first transfer nib N1 as shown in FIG. 1.
Subsequently, another electrostatic latent image for a second color is formed by charging and exposing the photosensitive drum 10. If a second color is magenta, as shown in FIG. 4, a developing unit 13-M having a magenta color accesses the photosensitive drum 10, thereby forming the electrostatic latent image. The developed magenta image is overlapped with the developed yellow image and transferred on the transfer belt 14 on which the yellow image was transferred. In the same way, another image having a cyan color as a third color, and another image having a black color as a fourth color are developed and transferred, thereby forming the color image having a desired color on the transfer belt 14. After that, the color image is transferred onto the paper supplied to a second transfer nib N2 between the transfer belt 14 and the second transfer roller 14b and is heated and pressed through the fusing unit 15 and is permanently fused on the paper.
Likewise, since the electrostatic latent image formed on the photosensitive drum 10 is developed while the four developing units 13-Y, 13-M, 13-C, and 13-K sequentially or selectively contact and/or separate from the photosensitive drum 10, shock may be applied to the photosensitive drum 10 whenever the developing rollers 13a of the developing units 13-Y, 13-M, 13-C, and 13-K collide with the photosensitive drum 10. In this case, jitter of the developed image may occur at the time of collision of the developing units 13-Y, 13-M, 13-C, and 13-K and the photosensitive drum 10.
An angular velocity of the developing rollers 13a of the four developing units 13-Y, 13-M, 13-C, and 13-K is larger than that of the photosensitive drum 10. Due to this angular velocity difference, a shock applied to the photosensitive drum 10 increases when the developing roller 13a contacts the photosensitive drum 10 to develop the electrostatic latent image. In general, a photosensitive layer is coated on an aluminum material to form the photosensitive drum 10, and the developing roller 13a formed of a rubber material has a predetermined hardness. Thus, if a hardness of the rubber material is reduced, the shock due to the contact may be slightly reduced.
However, an angular velocity difference is set such that toner T is supplied from the developing roller 13a to the photosensitive drum 10. When the developing roller 13a contacts the photosensitive drum 10, the developing units 13-Y, 13-M, 13-C, and 13-K are instantaneously pushed against the photosensitive drum 10, registration defects where overlapped layers of developed images of the colors are dislocated, and the jitter occurs. As a result, a resolution of the color image deteriorates. In addition, due to the contact shock, the toner T is scattered in the apparatus, and thus peripheral devices of the apparatus may be contaminated. Problems caused by the angular velocity difference may occur whenever the developing roller 13a contacts the photosensitive drum 10 and is separated from the photosensitive drum 10. These problems can be slightly reduced by limiting the angular velocity of the developing roller 13a. However, if the angular velocity of the developing roller 13a is too small, the toner T is not sufficiently supplied to the photosensitive drum 10 to develop the latent electrostatic image, and thus a quality of the color image deteriorates.
As mentioned above, since the photosensitive drum 10 is a rigid body, and since the leaf spring 13c formed in the developing units 13-Y, 13-M, 13-C, and 13-K has a large spring elasticity coefficient, when the photosensitive drum 10 contacts the developing units 13-Y, 13-M, 13-C, and 13-K, a repulsive force is strongly generated. As a result, a pressure difference between the photosensitive drum 10 and each of the developing units 13-Y, 13-M, 13-C, and 13-K occurs, and thus it is difficult that the photosensitive drum 10 stably contacts the developing roller 13a. Even though the photosensitive drum 10 stably contacts the developing roller 13a, the shock caused when the photosensitive drum 10 contacts the developing roller 13a is transferred to the photosensitive drum 10, and thus the jitter and bending of the color image occur.
Accordingly, in order to form a clear and a high quality image, it is required to provide a method of removing the above causes of image defects while maintaining a proper amount of the toner T supplied to the photosensitive drum 10 in the development step operation.
To solve the above and other problems, it is an object of the present invention to provide an apparatus for forming a color image using electrophotography. The apparatus reduces a shock and a pressure difference between a photosensitive drum and each developing roller to form a high quality image.
Additional objects and advantageous of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
Accordingly, to achieve the above and other objects, there is provided an apparatus for forming a color image using electrophotography. The apparatus includes a photosensitive drum on which a predetermined electrostatic latent image is formed, a plurality of developing units each including a developing roller supplying toner to a surface of the photosensitive drum to develop the electrostatic latent image, and pressure cams sequentially or selectively pressing corresponding ones of the developing units against the photosensitive drum when the electrostatic latent image is developed.
A bracket slide includes tension springs generating the same normal force in a predetermined range regardless of relative positions of the developing units with respect to the photosensitive drum. The same normal force is applied to the photosensitive drum from the developing roller while reducing a shock occurring when the developing roller contacts the photosensitive drum.
According to an aspect of the present invention, the tension spring of each developing unit has the same material, but has a different thickness from that of another developing unit. On the contrary, if the tension spring of each developing unit has the same thickness as that of another developing unit, the tension spring of each developing unit has a different material from that of another developing unit.
In the apparatus for forming a color image using electrophotography, the shock caused when the photosensitive drum contacts the developing roller can be slightly absorbed, and thus jitter or bending can be removed or minimized in a development operation. In addition, the same normal force applied to the photosensitive drum from the developing rollers regardless of positions of the developing rollers can be maintained, and thus the deterioration of an image quality due to the pressure difference can be prevented. Further, since the shock between the photosensitive drum and the developing roller is reduced, the developing roller can smoothly contact and/or be separated from the photosensitive drum, and the scattering of the toner caused when the developing roller contacts and/or is separated from the photosensitive drum can be minimized.